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tun - tunnel software network interface
The tun interface is a software loopback mechanism that can be loosely
described as the network interface analog of the pty(4), that is, tun
does for network interfaces what the pty(4) driver does for terminals.
The tun driver, like the pty(4) driver, provides two interfaces: an
interface like the usual facility it is simulating (a network interface
in the case of tun, or a terminal for pty(4)), and a character-special
device “control” interface.
The network interfaces are named “tun0”, “tun1”, etc., one for each
control device that has been opened. These network interfaces persist
until the if_tun.ko module is unloaded (if tun is built into your kernel,
the network interfaces cannot be removed).
The tun interface permits opens on the special control device /dev/tun.
When this device is opened, tun will return a handle for the lowest
unused tun device (use devname(3) to determine which). Control devices
(once successfully opened) persist until if_tun.ko is unloaded in the
same way that network interfaces persist (see above).
Each interface supports the usual network-interface ioctl(2)s, such as
SIOCAIFADDR and thus can be used with ifconfig(8) like any other
interface. At boot time, they are POINTOPOINT interfaces, but this can
be changed; see the description of the control device, below. When the
system chooses to transmit a packet on the network interface, the packet
can be read from the control device (it appears as “input” there);
writing a packet to the control device generates an input packet on the
network interface, as if the (non-existent) hardware had just received
The tunnel device (/dev/tunN) is exclusive-open (it cannot be opened if
it is already open). A read(2) call will return an error (EHOSTDOWN) if
the interface is not “ready” (which means that the control device is open
and the interface’s address has been set).
Once the interface is ready, read(2) will return a packet if one is
available; if not, it will either block until one is or return
EWOULDBLOCK, depending on whether non-blocking I/O has been enabled. If
the packet is longer than is allowed for in the buffer passed to read(2),
the extra data will be silently dropped.
If the TUNSLMODE ioctl has been set, packets read from the control device
will be prepended with the destination address as presented to the
network interface output routine, tunoutput(). The destination address
is in struct sockaddr format. The actual length of the prepended address
is in the member sa_len. If the TUNSIFHEAD ioctl has been set, packets
will be prepended with a four byte address family in network byte order.
TUNSLMODE and TUNSIFHEAD are mutually exclusive. In any case, the packet
data follows immediately.
A write(2) call passes a packet in to be “received” on the pseudo-
interface. If the TUNSIFHEAD ioctl has been set, the address family must
be prepended, otherwise the packet is assumed to be of type AF_INET.
Each write(2) call supplies exactly one packet; the packet length is
taken from the amount of data provided to write(2) (minus any supplied
address family). Writes will not block; if the packet cannot be accepted
for a transient reason (e.g., no buffer space available), it is silently
dropped; if the reason is not transient (e.g., packet too large), an
error is returned.
The following ioctl(2) calls are supported (defined in
TUNSDEBUG The argument should be a pointer to an int; this sets the
internal debugging variable to that value. What, if
anything, this variable controls is not documented here; see
the source code.
TUNGDEBUG The argument should be a pointer to an int; this stores the
internal debugging variable’s value into it.
TUNSIFINFO The argument should be a pointer to an struct tuninfo and
allows setting the MTU, the type, and the baudrate of the
tunnel device. The struct tuninfo is declared in
The use of this ioctl is restricted to the super-user.
TUNGIFINFO The argument should be a pointer to an struct tuninfo, where
the current MTU, type, and baudrate will be stored.
TUNSIFMODE The argument should be a pointer to an int; its value must be
either IFF_POINTOPOINT or IFF_BROADCAST and should have
IFF_MULTICAST OR’d into the value if multicast support is
required. The type of the corresponding “tunN” interface is
set to the supplied type. If the value is outside the above
range, an EINVAL error is returned. The interface must be
down at the time; if it is up, an EBUSY error is returned.
TUNSLMODE The argument should be a pointer to an int; a non-zero value
turns off “multi-af” mode and turns on “link-layer” mode,
causing packets read from the tunnel device to be prepended
with the network destination address (see above).
TUNSIFPID Will set the pid owning the tunnel device to the current
TUNSIFHEAD The argument should be a pointer to an int; a non-zero value
turns off “link-layer” mode, and enables “multi-af” mode,
where every packet is preceded with a four byte address
TUNGIFHEAD The argument should be a pointer to an int; the ioctl sets
the value to one if the device is in “multi-af” mode, and
FIONBIO Turn non-blocking I/O for reads off or on, according as the
argument int’s value is or is not zero. (Writes are always
FIOASYNC Turn asynchronous I/O for reads (i.e., generation of SIGIO
when data is available to be read) off or on, according as
the argument int’s value is or is not zero.
FIONREAD If any packets are queued to be read, store the size of the
first one into the argument int; otherwise, store zero.
TIOCSPGRP Set the process group to receive SIGIO signals, when
asynchronous I/O is enabled, to the argument int value.
TIOCGPGRP Retrieve the process group value for SIGIO signals into the
argument int value.
The control device also supports select(2) for read; selecting for write
is pointless, and always succeeds, since writes are always non-blocking.
On the last close of the data device, by default, the interface is
brought down (as if with ifconfig tunN down). All queued packets are
thrown away. If the interface is up when the data device is not open
output packets are always thrown away rather than letting them pile up.
ioctl(2), read(2), select(2), write(2), devname(3), inet(4), intro(4),
This manual page was originally obtained from NetBSD.